This invention relates generally to the repair of gas turbine engines and more particularly to the repair of turbine nozzle segments used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. Aircraft engines typically include a stationary turbine nozzle disposed at the outlet of the combustor. The nozzle includes a plurality of circumferentially spaced apart vanes that channel combustion gases into a turbine rotor located downstream of the nozzle. Turbine nozzles are typically segmented around the circumference thereof with each nozzle segment having one or more nozzle vanes disposed between inner and outer bands that define the radial flowpath boundaries for the hot combustion gases flowing through the nozzle.
Nozzle segments are exposed during operation to a high temperature, corrosive gas stream that limits the effective service life of these components. Accordingly, nozzle segments are typically fabricated from high temperature cobalt or nickel-based superalloys and are often coated with corrosion and/or heat resistant materials. Furthermore, nozzle segments are ordinarily cooled internally with cooling air extracted from the compressor to prolong service life. Even with such efforts, portions of the nozzle segments, particularly the vanes, can become cracked, corroded, and otherwise damaged such that the nozzle segments must be either repaired or replaced to maintain safe, efficient engine operation. Because nozzle segments are complex in design, are made of relatively expensive materials, and are expensive to manufacture, it is generally more desirable to repair them if possible.
Existing repair processes include techniques such as crack repair and dimensional restoration of airfoil surfaces. However, such existing repairs are limited by local distortion and under minimum wall thicknesses, which are exceeded as a result of repeated repair and chemical stripping processes. Thus, nozzle segments may become damaged to the point where they cannot be repaired by known repair processes. In turbine nozzle segments having two or more vanes, it often occurs that one of the vanes is repairable while the other vane is non-repairable. To avoid scrapping the entire nozzle segment in such a situation, techniques for salvaging the repairable portion of the nozzle segment have been sought.
One such technique is described in U.S. Pat. No. 4,176,433 issued Dec. 4, 1979 to Jack W. Lee, et al. This patent discloses a method of repairing nozzle segments (referred to therein as turbine vane clusters) in which the repairable vane from a damaged segment is separated from the non-repairable portion of the segment. The salvaged vane is then combined with a complementary repairable vane that has been similarly salvaged from another damaged segment. While this technique salvages repairable vanes that would otherwise be scrapped, the service life of the repaired nozzle segment is not prolonged very long because it contains used vanes that are limited in the number of future repairs that can be made. Furthermore, this technique is viable only as long as suitably complementary salvaged vanes are available to combine.
Accordingly, there is a need for a method of repairing turbine nozzle segments in which repairable vanes are salvaged in such a manner that the service life of the nozzle segment is greatly enhanced.
The above-mentioned need is met by the present invention which provides a method of repairing a turbine nozzle segment having at least two vanes disposed between outer and inner bands. The method includes the steps of separating the nozzle segment into a first singlet containing a repairable vane and a second singlet containing a non-repairable vane, and joining the first singlet to a newly manufactured singlet having a configuration that is similar to the second singlet.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.